1. Field of the Invention
The present invention relates generally to radiant heaters and more specifically to heating coils for use in such heaters.
2. Description of the Prior Art
Radiant heaters, which generate heat by passing an electrical current through a high resistance element, have become common place. However, new applications are continually being devised. With the dwindling supply of fossil fuels and their associated spiraling costs, more homes are using electrical radiant heaters as their primary or secondary heating source. Portable electric space heaters have also seen an increase in popularity. Large commercial space heaters for warehouses, garages and the like are also more common. Hand dryers, like those found in many public restrooms, and hair dryers, which are common in the home, also require radiant heater elements. With the advent of electrically powered cars, it is expected that highly efficient electric heating elements to heat the passenger compartment will be needed.
The electric current passes through a resistive heating element; in one form the heating element is exposed to the ambient, and in another form it is protected within a tube such as quartz or metal. Quartz heater tubes utilizing high temperature heating elements are common in the food service, graphic arts, and the industrial processing field. Applications in which quartz heaters can be found include very high speed drying of print, broiling and baking of foods in restaurants, drying ceramics and sealing plastics (e.g., bag forming processes). Quartz heaters are also replacing normal heating elements in stove stop ovens.
A typical heating tube consists of a high resistance wire wrapped in a helical configuration. The respective free ends of the helix are connected to a copper or other electrically conductive metal that serves as a common terminal point for that end. The helically configurative element is often suspended within a quartz or metal tube. The tube may be capped with ceramic end pieces or caps, and the helical heating element is held in tension by the end caps.
Some quartz heating tubes are vacuum sealed and may contain an inert gas. Frequently, special control circuitry is required because of an initial in-rush current when these devices are first activated.
A typical helical heating element may be, for example, from three inches to seventy-two inches long and from 0.250 inch to one inch in diameter. Generally, the helical structure is extended to space each coil from the adjacent coils. The wire has a diameter as required and is made of an alloy of iron, chromium, aluminum and cobalt which has a recommended operating temperature range that extends only to about 900 degrees centigrade. Though the alloy's melting point may exceed 900 degrees centigrade (e.g., 1280 degrees), the prior helical heating coil cannot effectively operate at such high temperatures.
In a typical commercial application, such as a cooking oven, heating elements are connected in an electrical series configuration. The size and wattage of the heating elements are designed for that particular oven.
The heating element is usually connected to an external terminal mounted on the tube's end caps. For example, a conically shaped termination for use in spring loaded sockets is often used. Studs, nuts, and pigtails are also common terminals. The terminal configuration depends on the application.
Quartz tubing has become increasingly popular to protect the heating element since it is durable and transparent to infrared radiation. The quartz tubing may be clear, semi-translucent or translucent.